1. Field of the Invention
The present invention relates to electron microscopes and, more specifically, to an improved emitter assembly and a concentric beam passage arrangement for use in electron microscopes.
2. Prior Art
Electron microscopes require a relatively high vacuum in order to operate properly. The electron beam sources used to produce the beam cannot operate properly unless maintained in a relatively high vacuum. Atmospheric gas molecules within a gun assembly will be ionized by the electron beam and the liberated electrons and ions will flow to the anode and cathode, respectively. This may result in current instability as well as potentially severe and damaging arcing. Even at low concentrations, the acceleration of ions into the cathode results in a shortened operational life of the gun assembly. Consequently, a relatively high vacuum is required within the gun assembly to protect the emitter.
The electron beam can be scattered by the presence of molecules within the beam path. Consequently, to maintain a highly focused beam, such scattering must be minimized by maintaining a relatively high vacuum within the beam passage. Furthermore, the possibilities of system or sample contamination due to the polymerization of volatile contaminants require the presence of a vacuum for proper operation.
Several early electron microscopes simply placed the electromagnetic focusing elements within the vacuum region. This arrangement became undesirable due to heat conduction, outgasing and other problems associated with placing the electromagnetic focusing elements within the vacuum region. Another design employed inner walls within portions of the electromagnetic focusing elements to form an inner vacuum passage. This design resulted in a complex series of vacuum seals. In both of these prior art designs, the electromagnetic elements were intimately connected with the vacuum system. Consequently, cleaning procedures required disassembly of the entire optical column. Both of these arrangements held serious drawbacks for electron microscopes.
A significant innovation took place with the development of the column liner tube. A column liner tube is generally a conducting, nonmagnetic tube which is placed through the central apertures of the various electromagnetic lenses and deflecting elements. The column liner tube is evacuated to permit the passage of the electron beam. Spray baffles may be typically located within the column liner tube to obstruct the passage of stray electrons which are scattered from the walls of the tube. Furthermore, a beam shaping aperture may be mounted within the tube. The principal virtue of this construction is that the electromagnetic elements are now located on the outside of the vacuum region. This permits a simpler, more compact construction and greatly facilitates the cleaning of the electron column. The column liner tube is typically made to be removable to further facilitate the cleaning. The use of a removable column liner tube is used in a majority of modern electron microscopes, examples of which are shown in U.S. Pat. Nos. 3,787,696 and 3,927,321.
The column liner tube of the prior art places several requirements on the liner tube itself. First, the liner tube must be capable of forming a vacuum-tight seal with the remainder of the microscope system. Furthermore, it must be completely conductive and must accurately maintain the position of the spray baffles and beam shaping apertures which are supported within the tube. Additionally, it must be capable of easy disassembly for cleaning. These multiple requirements result in a liner tube which must be precisely manufactured and assembled and is somewhat delicate to handle. Furthermore, the mating vacuum sealing elements of the column assembly experience wear due to the removability of the column liner tube and must be periodically serviced to maintain the proper sealing engagement.
The emitter assembly of known thermionic electron microscopes consists of a heated cathode which is mounted immediately behind a bias conductive structure known as the grid in which a central orifice is provided through which the electrons are emitted. Heating of the cathode causes electrons to be ejected from the cathode surface via thermal emission. Due to the negative electric potential of the emitter assembly, these electrons are then accelerated toward an adjacent anode structure which is maintained at a ground potential. The electrons pass through the anode structure into the beam passage of the microscope. The kinetic energy to which the electrons are accelerated is equal to the electrical potential (i.e., the beam voltage) maintained between the cathode and the anode.
The cathode is commonly a length of thin tungsten wire which is held between two mounting poles which are embedded in an insulating base made of glass or ceramic. The filament wire is typically bent to form a sharp point at the central location from which the electrons will be emitted. A number of alternative cathode materials, for example single crystal lanthanum hexaboride (LaB.sub.6) or cerium hexaboride (CeB.sub.6), are commonly used in place of the filament wire. The cathode and the supporting structure are referred to as the filament. Certain electron beams require the operation of the cathode at elevated temperatures. Evaporation of the cathode takes place at these elevated temperatures, and eventually the cathode will break or be depleted, requiring replacement of the filament. Conventional electron microscopes using a tungsten wire cathode are generally designed to achieve a cathode life of twenty to eighty hours of operation. Because of this, replacement of the filament is a common operation in such electron microscopes.
The grid plays an important role in controlling and shaping the emissions of the electron source. The grid is operated at a negative potential relative to the cathode so as to suppress the emissions of the cathode except at the very tip of the cathode where the potential field of the anode intrudes through an orifice in the grid. Consequently, by adjusting the suppression voltage on the grid (i.e., the bias voltage) the emission from the cathode can be confined to the small region at the tip of the cathode. The grid opening and the anode create an electrostatic lens which focuses the emitted electrons into a source spot. This source spot, also known as the crossover image, is further modified by the subsequent electromagnetic lenses. The quality of the image obtained from an electron microscope is intrinsically dependent upon the compactness and intensity of the source spot.
In order to obtain an appropriate source spot, it is necessary for the components of the emitter to be properly configured. Specifically, the grid orifice must be closely spaced and symmetrical relative to the cathode tip. Conventional electron microscopes typically use a grid orifice of one to two millimeters in diameter with the cathode tip set back somewhat less than this diameter. There are several practical requirements for the design of an emitter assembly as a result. First, the distance from the tip of the cathode to the grid must be accurately established. Second, the grid orifice must be accurately centered over the tip of the cathode. Third, the thickness of the grid adjacent to the orifice must be sufficiently thin to avoid contact between the cathode and the grid. Finally, the grid orifice must be highly uniform, circular and free of rough edges or insulating contaminants.
Cathode material evaporated from the cathode during operation of the emitter assembly results in the deposit of cathode material on the interior surface of the emitter assembly. These deposits must be periodically removed. Removing these deposits is a time-consuming operation requiring skill and care since improper handling can damage the orifice region. Because of the critical role in defining the performance of the electron microscope and the practical requirements for the cleanability, accurate alignment and other requirements of the emitter assembly, the emitter assemblies of the prior art have consistently been designed as high precision machined assemblies typically fabricated from stainless steel. As a result, the prior art emitter assemblies have been rather expensive to manufacture.
Various emitter assembly designs are known in the prior art. Some electron microscopes have been constructed such that the filament is plugged directly into sockets located in the microscope itself with the grid placed over it. However, the majority of electron microscopes employ a cartridge system in which the filament is mounted in a removable assembly which includes the grid. The cartridge system is more convenient in that the entire assembly can be cleaned, reloaded and aligned away from the microscope. However, the cartridge system is more complex, principally due to the necessity of providing reliable mating electrical connections to the filament.
U.S. Pat. No. 3,857,055 discloses a cartridge type emitter assembly. The emitter assembly clamps a filament unit between a back plate and an extended flange of a cap member. Adjustments between the filament and a grid orifice are provided by movements of the filament base through a limited range of motion provided by gaps within the filament base. The difficulty with the cartridge system disclosed in U.S. Pat. No. 3,857,055 is that a specialized filament is required for the construction of the emitter assembly and the design of the individual pieces require a certain amount of machining. Consequently, the cost of this emitter assembly is somewhat prohibitive.
The object of the present invention is to provide an emitter assembly which permits the fabrication of precisely aligned filament and grid members without the requirement of close tolerance parts. Another object of the present invention is to overcome the drawbacks of the emitter assemblies of the prior art discussed above.
A further object of the present invention is to provide an improved column liner construction which results in reduced fabrication costs and simplified maintenance. A further object of the present invention is to overcome the drawbacks of the conventional column liner tube construction of the prior art.